Recently, a lot of fundamental and applied researches, and developments have been extensively conducted on excellent tribological properties of functional carbon coatings such as DLC and CNx. These activities promoted the development of new coating technologies for functional carbon coatings. In Tribologist (Vol.58, No.8) published in 2013, special topic “Tribology of Hard Carbon coatings” was arranged, where Dr. Kano contributed an article titled with “Research Trend for Tribology of Hard Carbon Coatings”. In his article, which was written for the same purpose of this article, the latest trend of functional carbon coatings in the field of tribology before 2013 are well summarized. Therefore, in this article, I try to focus on the new things and progress in the same field after 2013 as much as possible.
In order to widely apply the diamond-like carbon (DLC) films to sliding elements, it is an urgent matter to elucidate the interaction with lubricant additives. This paper reviews the effects of lubricant additives on the tribological properties of DLC films under boundary lubrication regime according to recent researches. Friction modifiers (FMs) and anti-wear additives (AW) have different influences on the tribological performance of DLC films compared with metals. The design concept of DLC films based on “surface architecture” is introduced to develop DLC films tailored to conventional lubricant additives. However, the additional research is still needed to develop new lubricant additives tailored to DLC films for achieving a desired tribological performance and meeting environmental demands by revealing the tribochemical pathway of additives on DLC surfaces.
In general, tribological property of carbonaceous coating is thought promising material to reduce friction and wear of itself. The tribological property of carbonaceous coatings under lubrication is important. However its properties were varied by several techniques such as ultraviolet light irradiation, additives and/or contamination in lubrication. In industrial fields, the use of these coatings in true operation conditions, several particles derived from additives, contaminations and peeled material of coatings are expected to deteriorate its tribological property because of reentering into a contact point, therefore in-situ observation techniques of the contact point are thought to be important information. Thus, the manuscript introduce the tribological properties of hydrogenated (a-C:H), non-hydrogenated (a-C) and carbon nitride coating (CNx) with/without ultraviolet light irradiation effect under lubrication, the effect of texturing to eliminate imitation particles, and an intelligent Fluorescent Light Assisted Tribometer system was introduced to observe an act of contact point.
When using thin films to practical parts, not only the film properties such as hardness and lubricity, it is also necessary consider such adhesion and surface texture. There are dependent on the substrate, it is necessary to develop in consideration of properties of the substrate, such as hardness and fatigue strength. Surface modification is an effective way as the method to control the properties of the substrate material. It introduces FPB (Fine Particle Bombarding) as one of the surface modification methods. FPB is a kind of the shot peening, but is a method of projecting particles finer than normal at high speed. The projection of high-speed fine particles can bring large plastic deformation on the substrate surface. Plastic deformation causes giving of compressive remaining stress and formation of micro-dimples in a substrate material surface, and improves fatigue strength and the sliding properties.
Mechanical seals require sliding material combinations having lower friction coefficient and higher wear resistance in order to fulfill increasing demand for saving energy, space and maintenance costs. Carbon hard coatings such as diamond and DLC coatings are considered as some of the most promising choices for that. Furthermore, due to their excellent chemical stability, they are preferably used in various working fluids and applications so as to prevent corrosion or chemical reaction as well as sliding material wear. Severe damages due to electrochemical corrosion were found on conventional SiC sliding materials of a mechanical seal in BFP (boiler feed pump) of CWT (combined water treatment) after 500 hours of operation. A diamond coating was applied to the SiC sliding materials to prevent the damages due to electrochemical corrosion and they could operate without damages during more than three years. Excessive wear occurred to conventional SiC sliding materials of dry gas seals due to dry running during startup and shutdown in high-pressure compressor applications such as EOR (enhanced oil recovery) and CCS (Carbon Capture Storage). A DLC coating was applied to the SiC sliding materials to prevent the wear due to dry running and it could realize successful operation of dry gas seals. From the perspective of expanding the application of carbon hard coatings to mechanical seal sliding materials, this paper also discusses challenges such as mass productivity of them, control of characteristics of DLC coatings, effective surface roughness control of diamond coatings and applying them to surface textured sliding materials.
The rolling elements used in ball bearings are typically high-carbon chromium steel balls, although silicon nitride ceramic balls are also occasionally employed. Research on the use of ceramic balls in ball bearings has been conducted since 1980, but only silicon nitride is presently used since its strength and durability matches those of steel balls. In the motors of home electrical appliances, it grows popular that electrical damage to bearing, such as electrical pitting, is a cause for failure. Utilizing ceramic balls is known to be an effective countermeasure, but is rarely employed due to their high cost. However, since the bearing load on ball bearings used in the motors of home electric appliances is small, strength comparable to that of silicon nitride may not be necessary. In this study, the performance of a ball bearing (608) containing zirconia balls is experimentally evaluated under light load conditions (~20% of the basic dynamic load rating). The obtained data indicate that the bearing performance is acceptable under such conditions.
The contact type of tribocharge power generator was developed. This tribocharge power generator has a simple structure which two types of tribocharging films on electrode are laminated by silicon rubbers. That generated the volume density of electric power of 94µW/cm3 under the condition of 40Hz, 2G, and 55kPa. As the results, we confirmed that the tribocharge power generator was useful for the energy harvesting and sensing the acceleration.